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    SubjectRe: regression(?): starting with 2.6.21 sending packets became broken.
    Hello, David, Jan.

    В Сбт, 13/10/2007 в 19:59 +0100, David пишет:
    > Try
    > echo 0 > /proc/sys/net/ipv4/tcp_window_scaling
    > I bet you have broken router(s) between your machine and the problem
    > site(s).

    Thank you for your help, but it occurs that the problem comes from IMQ
    patch. I've reported that on imqlinux mailing list, but seems that it's
    closed for those who do not have yahoo ID so I copy my report there and
    answer here.

    ===========my investigation of the problem=================

    This bug was reported in our bugzilla ( ) but as
    I found that IMQ is the root of problems I wanted to share my experience

    Starting with 2.6.21 (or may be 2.6.20 as I have not tried it) kernel I
    have problem that most tcp based services freeze at some point of
    operation. I've noticed this first on ssh but then found out that at
    lease one other service
    became similarly. The problem sites somewhere in the kernel as I've
    compiled 2.6.19, 2.6.21, and 2.6.22 with the similar .config options (of
    course not exact, as some options does not exist in some kernels, but
    seems that
    enabled options are all the same) but I have this problem only with the
    21 and 22. I've tried to debug the problem a bit, but not a lot as that
    is production box working as linux based firewall/router.

    First I took tcpdump. Although ssh connection to the router is not
    always possible as it often hangs before I get into router, after some
    attempts ssh connection was established. On client computer I've started
    tcpdump and worked a bit until hang. tcpdump output showed me that when
    I press any keys the packets are sent to the server and proper ack are
    received. Later I found that all commands I enter blindly are executed
    on router but I receive no reply packets with some data in them (pure
    ack). That's why nothing happens on the screen and it looks like

    Now I've got to the router started ssh connection from router to some
    other server. It hanged too. I attached strace and found that ssh
    receive keyboard pressings (read() calls in the output) and writes them
    to the kernel (write() calls) but tcpdump on the router shows no
    packets. So packets enter kernel and lost somewhere inside.

    This problem was reproduced both on single core amd64 system and on x86
    system with hyper threading. So I suspect everybody could reproduce this
    problem. Just start `yes` which produce a lot of output and then press
    Ctrl+C to interupt. It hanged here somewhere at this moment.

    Suggestion "echo 0 > /proc/sys/net/ipv4/tcp_window_scaling" does not
    helped here.

    The end of the story is that if I localized that the problem sites in
    this IMQ patch.
    If I install clean gentoo sources then connection does not freeze and
    with this patch I have problems. BTW. This patch does not have this
    problem here too.

    Thank you for your attention. I think may be it's good idea to mark that
    patch as questionable on site?


    ======== vlad031 answer on linuximq mailing list===========

    Re: regression(?): starting with 2.6.21 sending packets became broken

    We allready know that ... almost everybody does ... that's why we have
    uploaded the good patches here:

    Andree isn't seem to take care of this project anymore...

    Please note that 2.6.23 kernel has a lot of bugs and we don't recommend
    using it yet as we had some imq problems related too (however, it doesnt
    seem to be from imq patch)



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